Synchronous requests that get a 304 clobber the cached response body.

third_party/WebKit/Source/core/fetch/ResourceLoader.cpp

 /*
  * Copyright (C) 2006, 2007, 2010, 2011 Apple Inc. All rights reserved.
  *           (C) 2007 Graham Dennis (graham.dennis@gmail.com)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 497 matching lines @@
             return;
         }
         didFail(0, errorOut);
         return;
     }
     didReceiveResponse(0, responseOut);
     if (m_state == Terminated)
         return;
     RefPtr<ResourceLoadInfo> resourceLoadInfo = responseOut.toResourceResponse().resourceLoadInfo();
     int64_t encodedDataLength = resourceLoadInfo ? resourceLoadInfo->encodedDataLength : WebURLLoaderClient::kUnknownEncodedDataLength;
-    m_fetcher->didReceiveData(m_resource, dataOut.data(), dataOut.size(), encodedDataLength);
-    m_resource->setResourceBuffer(dataOut);
+
+    // Follow the async case convention of not calling didReceiveData or
+    // appending data to m_resource if the response body is empty. Copying the
+    // empty buffer is a noop in most cases, but is destructive in the case of
+    // a 304, where it will overwrite the cached data we should be reusing.
+    if (dataOut.size()) {
+        m_fetcher->didReceiveData(m_resource, dataOut.data(), dataOut.size(), encodedDataLength);
+        m_resource->setResourceBuffer(dataOut);
+    }
     didFinishLoading(0, monotonicallyIncreasingTime(), encodedDataLength);
 }
 
 ResourceRequest& ResourceLoader::applyOptions(ResourceRequest& request) const
 {
     request.setAllowStoredCredentials(m_options.allowCredentials == AllowStoredCredentials);
     return request;
 }
 
 }